Move skills watcher to app-server (#21287)

## Why

Skills update notifications are app-server API behavior, but the watcher
lived in `codex-core` and surfaced through
`EventMsg::SkillsUpdateAvailable`. Moving the watcher out keeps core
focused on thread execution and lets app-server own both cache
invalidation and the `skills/changed` notification.

## What changed

- Added an app-server-owned skills watcher that watches local skill
roots, clears the shared skills cache, and emits `skills/changed`
directly.
- Registers skill watches from the common app-server thread listener
attach path, including direct starts, resumes, and app-server-observed
child or forked threads.
- Stores the `WatchRegistration` on `ThreadState`, so listener
replacement, thread teardown, idle unload, and app-server shutdown
deregister by dropping the RAII guard.
- Removed `EventMsg::SkillsUpdateAvailable`, the core watcher, and the
old core live-reload test.
- Extended the app-server skills change test to verify a cached skills
list is refreshed after a filesystem change without forcing reload.

## Validation

- `cargo check -p codex-core -p codex-app-server -p codex-mcp-server -p
codex-rollout -p codex-rollout-trace`
- `cargo test -p codex-app-server
skills_changed_notification_is_emitted_after_skill_change`

codex-rs/core/src/codex_delegate.rs

@@ -83,7 +83,6 @@ pub(crate) async fn run_codex_thread_interactive(
         skills_manager: Arc::clone(&parent_session.services.skills_manager),
         plugins_manager: Arc::clone(&parent_session.services.plugins_manager),
         mcp_manager: Arc::clone(&parent_session.services.mcp_manager),
-        skills_watcher: Arc::clone(&parent_session.services.skills_watcher),
         conversation_history: initial_history.unwrap_or(InitialHistory::New),
         session_source: SessionSource::SubAgent(subagent_source.clone()),
         thread_source: Some(ThreadSource::Subagent),